This invention relates generally to the field of optical amplifiers and more particularly to a double clad amplifying optical fibre.
An optical amplifier is a device that increases the amplitude of an input optical signal that is launched into the optical amplifying fibre together with pump light. When the optical signal at the input to the amplifier is monochromatic, the output signal is also monochromatic with the same frequency.
Conventional optical fibre amplifiers comprise a gain medium such as a single mode-glass optical fibre having a core doped with an active material or dopant excited by pumping the optical fibre with optical energy at predetermined wavelengths within an absorption band of the optical fibre dopant. Typically, the dopant is a rare earth such as erbium (Er) and or ytterbium (Yb).
In prior art double clad optical fibre amplifying systems, the single mode optical fibre has an inner cladding adjacent to the core, and an outer cladding adjacent to the inner cladding. Pump light is launched into the inner cladding of the optical fibre while the signal to be amplified is launched into the core.
As is well known in the art, the pump energy produces a population inversion in the dopant, and the input signal is amplified by stimulated emission to produce an amplified output signal, which is emitted from the other opposite end of the optical fibre. Such optical fibre amplifiers can be unidirectionally pumped with pump sources at one end or bi-directionally pumped with pump sources provided at opposite ends of the optical fibre, one co-propagating with the signal and the other counter-propagating with the signal. The employment of bi-directional pumps provides for higher power output and more power conversion efficiency in the optical fibre. Since U.S. Pat. No. 5,140,456 to Huber, it is well known that a co-propagating pumping configuration provides for lowest noise figure performance for erbium doped fibre amplifier (EDFA), and counter-propagating pumping provides for highest power conversion efficiency.
The optical fibre of the co-propagating fibre amplifier is preferably doped with erbium (Er3+). This provides an amplifier which functions well for boosting the energy of a signal having a wavelength of approximately 1550 nm using pumping energy in the wavelength range of 980 nm and/or 1480 nm.
Also known in the art, are optical isolators that are used throughout the amplifier to ensure unidirectional propagation of the optical signal at certain points in the signal path. For example, the optical input signal is passed through an optical isolator at the amplifier input and, again at the amplifier output to minimize amplified spontaneous emission (ASE) and noise coming from optical component reflections or from an optical device to which the amplifier output is coupled.
Also, of concern is the noise figure of the amplifier, which is generally defined as the ratio of the input signal to noise ratio to the amplifier output signal to noise ratio. In order to decrease the noise figure of an amplifier, the amplifier is typically pumped with pump energy that propagates in the same direction as the input signal propagation direction.
It is generally known that Er/Yb co-doped double clad fibre amplifier gives high power output. However, due to the presence of a double cladding and Yb dopant, the population inversion is lower than in the case of single mode laser pumped Er doped single mode optical fibre especially in the case of counter-propagating pumping. For this reason, noise figure is high and usually, a single mode pre-amplifying stage is provided to improve noise figure.
Amplifier pump sources are often in the form of laser diodes available as single mode or multi-mode diodes. Single mode laser diodes are effectively point sources, diffraction limited in their divergence in both axes. Multi-mode diodes typically have laser junctions in the form of short bundles, ranging between 10 to 200 microns long. Diffraction of multi-mode diodes is limited in the direction perpendicular to the junction, but they have non-diffraction limited divergence in the direction parallel to the laser junction. The emitting aperture of a multi-mode diode can be a single continuous stripe, a collection of short stripes or even a collection of single mode emitters electrically connected in parallel. The single mode pump laser referred to is a single stripe diode laser that has a single mode fiber pigtail. The multimode laser is a multi-stripe diode laser that has a multimode fiber pigtail. The core size of the multimode fiber is equal or close to the size of the first cladding of the double clad fiber mentioned above.
It is an object of this invention to provide an optical pumping scheme that will provide significant amplification of an optical signal with low noise figure at a relatively reasonable cost.
In accordance with the invention there is provided an optical amplifier for amplifying an input optical signal comprising a wave-guide having a core doped with a rare earth and an adjacent first cladding; a first laser pump coupled to the core; and, a second laser pump coupled to the first adjacent cladding.
In accordance with the instant invention, the first pump is a single mode laser pump and the second pump is a multi-mode laser pump; and the two pumps launch simultaneous optical signals into the core and into the first adjacent cladding.
In accordance with this invention, the simultaneous coupling of the single mode laser pump and the multi-mode laser pump allows counter propagation of the two launched pump signals.
In accordance with this invention, there is furnished a method of amplifying an optical signal comprising the steps of providing a double clad optical fibre having an inner cladding and having a core doped with a rare earth; and launching the optical signal into the core of the optical fibre while simultaneously launching a pump signal into the inner cladding and another pump signal into the core.